HHVM is an open-source virtual machine developed by Facebook and designed for executing programs written in Hack and PHP. It offers increased performance for PHP, most of the time. [...] Since Composer needs to perform some heavy computations in order to resolve the dependencies of a project, it makes sense to use HHVM. However, the heavy computations are mainly done when running composer update, or when the composer.lock file has not yet been generated so this is where you will see most of your gains in execution time.

With a bit more testing, this is shown to be true (about a 7 second difference). However, this is only on the "update". The "install" command actually takes longer inside of the HHVM instance, regardless of if the JIT (Just In Time) compiler is disabled or not.